The invention relates to a method and apparatus for expansion forming of tubing.
Various methods for expansion forming of tubing are known. For example, it is known to form frame members by pressurizing and expanding a tubular blank within a die cavity having a rectangular cross-section with rounded corners corresponding to the cross-section of the final frame member. This allows frame members to be produced with very accurate dimensions at high speed and at relatively low cost. Preferably, the expansion of the circumference of the blank is less than about 5%, so that frame members of excellent strength properties can be obtained using, for example, starting material tubular blanks of ordinary grades of steel. Commonly assigned U.S. Pat. No. 4,567,743 dated Feb. 4, 1986 and Re. 33990 dated Jul. 14, 1992, each in the name I. G. Cudini, disclose techniques for allowing a blank to be confined without pinching in a die having its die cavity circumference exceeding the circumference of the blank by no more than about 5%.
Various methods may be employed for sealing an end of a tubular blank to allow the interior to be pressurized. U.S. Pat. No. 2,837,810 in the name Ekholm, for example, discloses a circular plug having a resilient annular sealing element which is inserted inside one end of a tube to be expanded within a die, and the annular sealing element is compressed to seal on the inner circumference of the tube.
Copending and commonly assigned U.S. patent application Ser. No. 07/860,553 filed Mar. 30, 1992 (now U.S. Pat. No. 5,525,836) discloses tube sealing means having an elastomeric sealing element compressible between ring members to radially expand the sealing element into sealing engagement with the tube from a normal or relaxed position in which it is normally nested inwardly between the ring members. When such sealing arrangements are used for the expansion of tubes to form them into, for example, frame members having a rectangular or other elongated cross-section, it is necessary to provide a transition portion between the tube end receiving the sealing element and the inner elongated cross section portion. Applicant has found that if the angle of inclination of the wall of the transition portion relative to the tube axis is made too great, the transition portion tends to wrinkle longitudinally. The wrinkling tends to extend to the expansion formed portion and is not removed by the pressures ordinarily used for expansion and can render the frame member or other product unusable. As a result, with the known method, for a given starting material tube and a desired final product of, for example, rectangular section having given dimensions it is necessary to provide a transition portion of a certain minimum length. Since in many cases a frame member wholly of certain cross sections is desired, it is usually necessary to cut off the transition portion. With the known methods, the excessive length of the transition portions results in considerable waste of materials and resources.